The present invention relates to a process for improving the overall lifetime of optical recording media. More particularly this invention relates to a process for stabilizing optical information storage media containing polycarbonate substrates.
Typical substrate materials used in the manufacture of optical recording media include acrylic resins, methacrylic resins, polystyrene resins, polycarbonate resins, cellulose resins, polyester resins, polyamide resins, polyolefin resins, epoxy resins, polyimide resins, cellulose triacetate, cellulose acetate butylate, polymethylpentene, and polyethylene terephthalate. Among the most preferred substrates are the polycarbonate substrates, due to their excellent optical properties and physical stability. Despite the excellent properties of the substrate per se, however, degradation of the optical properties of the optical information medium over time, for whatever reason, can cut short the useful lifetime of the optical information medium in which the substrate is incorporated. Many different and complicated technologies have been used in an attempt to solve this problem and thereby extend the useful life of a medium.
For example, U.S. Pat. No. 4,571,314 to Suzuki et al discloses a process for producing a substrate for optical recording media which comprises coating at least one inner surface of a casting mold with a radiation-curable or thermosetting resin. The resin is then cured to form a coating film on the mold surface or surfaces. An epoxy resin composition comprising an alicyclic epoxy resin or a mixture of an alicyclic epoxy resin and an aromatic epoxy resin, an organic polybasic acid anhydride, a curing accelerator and an anti-discoloring (anti-oxidative) agent is then cast into the mold and heat-cured to transfer the coating film on the mold to the surface of the cured product of the epoxy resin composition, thereby forming a double or triple layer cast laminate. This process is alleged to provide a physically stable and scratch-resistant substrate more reliably than the more conventional process of first molding a polymethylmethacrylate or polycarbonate sutstrate and subjecting same to a surface-hardening treatment.
Other methods for increasing the chemical and mechanical stability of an optical disk structure have included the use of a protective layer (Japanese Pat. Application Laid Open Nos. 57-60542 and 57-66541), and the use of a complicated "sandwich structure".
Because of the potential value of stable optical information media, and in particular stable optical information media containing polycarbonate substrates, it would be advantageous if improved stability, in terms of extended useful life of the media, could be realized easily and inexpensively without the need for complicated and expensive structures, reaction steps or chemical treatments.
Accordingly, it is an object of the present invention to provide a simple and commercially viable method for improving the stability of optical information media using polycarbonate substrates.
Another object of the present invention is to provide an optical information storage medium exhibiting improved overall lifetime stability.
It is yet another object of the present invention to provide a process for preparing an optical information storage medium comprising a polycarbonate substrate which exhibits a longer useful life and which also exhibits excellent performance over time.
These and other objects, as well as the scope, nature, and utilization of the invention, will be apparent to those skilled in the art from the following description, the Drawing and the appended claims.